Space telescopes such as the Hubble and James Webb have either continuous or segmented mirrors as primary objectives, arranged in a Ritchey-Chretien-Cassegrain, a three mirror anastigmat Korsch, or similar system. As space telescopes become larger, certain disadvantages become apparent. Mass, and cost, increase at a rate between the square and cube of the aperture. The requirement for precision of segment fabrication, and precision of location and angular orientation of the mirror segments becomes more difficult to meet. A conventional telescope places heavy reliance on the objective to focus the incident beam to an image, requiring severe design constraints.
A type of large telescope that has been proposed to address some of these concerns is the diffractive, in which a thin diffractive phase controlling objective focuses light to an image sensor or to an inverse diffractive secondary focuser to form an image. An example is the Eyeglass concept (U.S. Pat. No. 6,219,185 Hyde). This type of instrument involves a lightweight objective made from large numbers of diffractive glass panels, deployed as origami-folded, which can handle about a factor 3 variation in wavelength without excessive loss of light transmitting capacity. Other examples of the diffractive type are Nautilus (University of Arizona College of Optical Sciences), the Koechlin Fresnel Imager, and Moire (DARPA), involving thin diffractive membrane objectives, with similar limitations on spectral bandwidth. Also of interest is the Cash Aragoscope, which diffracts light around the rim of an occluder disk, imaging at the optical axis mainly bright extended sources such as star surfaces. This device has wide spectral range and high resolution but low sensitivity.
In a different category the Labeyrie “hypertelescope” consists of a flotilla of independent mirror spacecraft that focus incident beams to an image sensor. In some Labeyrie versions, a single large mirror is formed from small reflective particles trapped along a parabolic surface by standing waves produced by a laser. Here, problems may arise from disturbing influences such as sunlight and electrostatic charges accumulated from solar wind. In versions involving the Hanbury Brown and Twiss intensity interferometry technique, a flotilla of independent telescopes is deployed, all aimed at the same distant object and measuring in real time the variation of light intensity from that object. The intensity information is sent to a central computer which uses an algorithm to construct an image.
In the present device there is a division of labor. Three components: the objective, a secondary mirror and an optical path corrector all share the work of focusing, reducing the design burden on each component. The present device offers the advantage of greatly reducing the mass of components to be transported to space. It also greatly increases the tolerances for error in placement and orientation of the segments of the objective. Another advantage is a wide field of view, comparable to refractors. The optical train is totally reflective throughout, the light not passing through any optical materials on its path to the image plane. Consequently, the most prominent advantage of the present device is a very wide spectral range from near UV to mid infrared, with high transmission efficiency throughout the range. The resulting versatility is of paramount importance for astronomy even more than for Earth observation.